The rate metabolism of mephenytoin to its metabolite, nirvanol, has not been determined although mephenytoin has been used for over 30 years in the treatment of seizures. Five epileptic patients were given single doses of 50 and 400 mg mephenytoin. The plasma levels of both drugs were measured by GC-MS-mass-fragmentography. Mephenytoin is rapidly absorbed and disappears from plasma with a plasma t1/2 range of 8 to 24 hours. Nirvanol appears in the plasma soon after ingestion of mephenytoin and reach peak plasma levels in 24 to 36 hours. Nirvanol then disappears from plasma with a plasma half-life of 84-144 hours. Chronic administration of mephenytoin (400 mg/day) produces steady-state levels of nirvanol much higher than those for mephenytoin. Steady-state levels of nirvanol are achieved in 14-21 days whereas mephenytion steady-state levels are reached in 6 days following chronic administration.